Technology

The thing about printers, is that unlike most other common modern computer peripherals, they're mechanically complex, and with precise tolerances at that, given that they have to be able to feed paper through the system with pure friction.

A daisy wheel or other character-based impact printer could probably be produced in a few months' steady work by a sufficiently stubborn and enthusiastic DIY type, but almost no one would buy or build one because it's a step backward in terms of output results—text output in the font face and point size on the daisy wheel only. There might be a niche for them as ruggedized, easy-to-repair printers in developing countries or other awkward situations.

A laser printer would probably take a couple of years of development to reach reasonable feature parity with current closed-source offerings, and then you'd have to set up a production line. The problem there is likely to be funding: you're going to have a hard time finding an investor to dump money into creating something that already exists, and it's difficult to spin "open hardware" as a selling point to businessmen. So you'd need to be independently wealthy, or convince someone who is to back the project.

Inkjets? Not a prayer, I don't think. Too much expensive-to-produce microminiaturized equipment involved in the ink-spraying. It's also the newest of the technologies, so the most likely to have a patent minefield lying in wait for the unwary. And existing ones are usually sold below cost, so it would be difficult for such a project to break even.